1. Field of the Invention
The present invention relates to a method of forming a silicon-based thin film, a silicon-based thin film, and a photovoltaic element such as a solar cell formed by depositing one or more units of pin junctions, a sensor and the like.
2. Related Background Art
A high frequency plasma CVD method is one of excellent methods for mass production of silicon-based thin films in terms of capability of achieving surface area enlargement and low temperature formation and also improving process through-put. Regarding to a solar cell as an example of application of a silicon-based thin film to a product, as compared with existing energy utilizing fossil fuel, a solar cell using a silicon-based thin film has advantages that its energy source is inexhaustible and that its power generation process is clean. In order to make wide use of it, however, a cost has to be reduced. For that, establishment of technology relevant to improvement of a film forming rate using the high frequency plasma CVD method is one of important technological problems to be solved.
Regarding a high frequency plasma CVD method with an increased film forming rate, Japanese Patent Publication No. 7-105354 discloses that, when paying attention to the relation between frequency f of high frequency in a range from 25 to 150 MHz and distance d which is a distance (cm) between a substrate and an electrode, methods carried out in a range of f/d from 30 to 100 MHz/cm are preferable and especially methods carried out in a range of d from 1 to 3 cm and in a range of the pressure from 0.1 to 0.5 mbar are preferable.
Further, regarding a method of forming a crystalline silicon-based thin film, Japanese Patent Application Laid-Open No. 11-330520 discloses that it is possible to carry out growth of a silicon-based thin film at a high film-forming rate under the condition that the reaction chamber contains a silane-based gas and hydrogen gas and has the pressure set at 5 Torr or higher and a distance between a substrate and an electrode is 1 cm or shorter and also discloses a photovoltaic device using the film has a high conversion efficiency.
However, as described above, the former already disclosed high frequency plasma CVD method is a method taking account of the value of f/d, and the former discloses that a high quality film with a high deposition rate and a low defect density can be formed especially when d is small. However, 1 cm is the minimum for the distance d owing to the effect of other phenomena, and technique for high rate film formation for smaller distance is not mentioned. On the other hand, regarding the latter production method, the distance between the deposition face of a substrate and the opposing surface of an electrode is within 1 cm and further the substrate is installed on a discharge electrode and it is a technique limited for batch type formation.
Further, an evaluation of properties of a film for the former is carried out based on evaluation of defect density in mostly a single layer state and no particular description is given regarding damages giving an underlayer and effects on adhesion and resistance to environments in the case of application of the former method to the stacking of the film at the time of device formation. Also, regarding the film itself, it is restricted to amorphous silicon and there is no reference to a film containing crystal phase.
Further, in the case of the latter, the source gases are mainly a silane-based gas and hydrogen gas and the attention is paid only to the flow rate of the silane-based gas and hydrogen gas, and there is no description referring the effect of the residence time defined by the capacity of the discharge space and the flow rates of raw materials gases on the film quality and production of reaction byproducts.